There are 2 schools of thought regarding this:

1). Add a field(s) for donations (similar to the fields in your donations settings) as membership fields.  If you add the field and make it optional, it is NOT something that will come up at the time of their renewal UNLESS they put in a previous amount in that field. When the invoices are created for renewal, and the member has not chosen to donate through their profile field, it will not show that amount in the renewal invoice and will not be a part of their renewal amount due. AND those fields need to be designated to be added to the renewal amount each year. However, if you receive a donation this way AND need to track it separately, then you will have to run a report to pull out all the donations from you dues received.

2). The easier way to do all of this is to set up a donation page and put a link into your Default Member's Renewal Invoice suggesting that if the wish to make a donation to click on the link provided. This way the donation will be applied properly in WA, they will get a receipt that they can use for a tax deduction and they can then proceed to pay their dues at the same time.